The formation of a contact plug on a semiconductor device is well known in the art. A common process for forming a contact plug comprises applying a layer of silicon dioxide (SiO.sub.2) to a substrate surface of polysilicon, silicon (Si) or metal. A pattern is then formed on the SiO.sub.2 by a standard photoresist process, and a contact or hole is etched into the SiO.sub.2.
Tungsten hexafluoride (WF.sub.6) plus hydrogen (H.sub.2) is used in a chemical vapor deposition (CVD) process, as is well known in the art, to deposit tungsten (W) into the contact. For the CVD process to work, the H.sub.2 must be dissociated into atomic H which is reactive with the WF.sub.6. The combination of WF.sub.6 and atomic H creates W plus hydrogen fluoride (HF). The HF is volatile and, therefore, is rapidly removed as a gas while the W is deposited into the contact.
If the substrate comprises a metallic substance, the metal will provide a catalyst to initiate the disassociation for the H.sub.2. Thus, the W from WF.sub.6 is allowed to attach to the metallic surface and deposit in layers within the contact. The layers are formed generally horizontal from the bottom of the contact to the top of the contact. Horizontal layering is caused because W is selective to the SiO.sub.2, meaning it will not deposit on the SiO.sub.2 sidewalls. However, the layering process takes so long to completely fill the contact that while the layering is proceeding, the top surface of the SiO.sub.2 also being bombarded with the reaction products from the WF.sub.6 and the H.sub.2. Eventually, the SiO.sub.2 surface may become damaged or contaminated, allowing tungsten nucleation to begin where it is not wanted. This forms W bumps on the surface which may create intralevel shorts in subsequent patterned metal layers.
If the substrate is comprised of Si, or certain metal silicides, the Si does not dissociate the H.sub.2 as well as a metal does. Unfortunately, the WF.sub.6 will also react with Si to create silicon tetrafluoride (SiF.sub.4) which is a gas. The SiF.sub.4 creation removes Si from the surface upon which the contact plug is to be built. Thus, as W is deposited, Si is also removed vertically as well as horizontally from under the SiO.sub.2 (encroachment). The removal of the Si in both directions is undesirable as it may cause shorting of p/n junctions positioned near the contact.
Due to the layering effect of the W (W being selective to SiO.sub.2), the W deposition is a time consuming process. The longer the deposition takes, the more likelihood there is of W also being deposited on the surface of the SiO.sub.2. Thus, there is a need for a way to enhance nonselective W deposition in a contact on a substrate without damaging the surrounding surfaces.